1. Field of the Invention
This invention relates generally to computer data storage systems consisting of arrays of individual disks and, more particularly, to an arrangement of disks and controllers that improves data storage reliability through redundancy.
2. Discussion of the Related Art
Small magnetic disk storage devices with hundred megabyte capacities offer cost and power advantages over larger disks having capacities on the order of gigabytes. Because the smaller disks offer the same I/O bandwidth as the larger ones, several smaller disks can provide greater data bandwidth than one larger disk of equivalent capacity. Because of this advantage, computer disk subsystems that consist of arrays of small disks are desirable.
An array of n small disks, each having the same mean time between failures, MTBF1, has an overall mean time between failures of MTBF1/n, assuming that a single disk failure is an array failure and that disk failures in the array are statistically independent. Thus, an array of twenty small disks should fail twenty times more often than a single large disk of similar quality and technology. To overcome this problem, small disk arrays can be organized to improve reliability through some sort of redundancy, that is, by decoupling single disk failure from array failure.
Practitioners in the art propose a variety of parity schemes for error correction in a redundant array of small disks. For example, an additional disk can serve as a parity disk for each group of m data disks, so that each parity disk sector contains the "exclusive-or" (XOR) of the contents of all corresponding sectors on the m data disks. If a data disk sector fails, its contents may then be reconstructed by XORing the (m-1) other data sectors with the corresponding parity sector. Schemes of this type are disclosed in U.S. Pat. No. 4,092,732 issued to N. K. Ouchi. Similar techniques for data redundancy in disk arrays are also discussed in a paper by D. A. Patterson et al., "A Case for Redundant Arrays of Inexpensive Disks (RAID)", Proceedings of the ACM SIGMOD Conference (Jun. 1-3, 1988), pp. 109-116.
In redundant arrays of small disks, it is also well-known to reserve some disks unused in a "hot standby" mode so that they may be switched into the array to immediately replace a failing data disk without system interruption.
Disk drive and disk controller interconnection architecture significantly influences the performance and reliability of small disk arrays. Because the controllers themselves and the interconnection cabling to the disks are also subject to failure, acceptable reliability may require some form of controller and interconnect cabling redundancy as well.
M. Schulze et al. ("How Reliable is a RAID?", Proceedings of the Spring COMPCON (1989), pp. 118-123) consider controller and disk interconnection architecture in the context of the Small Computer Systems Interconnect (SCSI) Standard. They propose a scheme for organizing a redundant disk array into multiple strings of disk drives, each string having one controller and one SCSI interconnect path with a plurality of drives sharing the single controller and path. Each parity group is laid out across these strings so that no two disks within a parity group are from the same string (FIG. 1). Thus, Schulze et al. teach a scheme that allows data recovery by parity regeneration from controller or path failure because a single such failure affects no more than one disk in each parity group. However, for larger parity groups, many controllers are required, increasing costs. Schulze et al. also propose an even more costly scheme using duplicate controllers, but do not consider accompanying means for redundancy in the SCSI interconnections that join the controllers to the disks.
FIG. 2 shows an alternative disk array architecture in the related art wherein each disk drive is dual-ported, thereby being adapted for independent coupling to either of two controllers. Each of two controllers is connected to a single dual-ported disk by an independent interconnect path. The disk drives in the array are grouped into strings where each string shares the same pair of controllers. Each parity group lies entirely within a single string. With this arrangement, an entire array could lie within a single string, requiring no more than two controllers, thereby reducing cost. However, because each parity group is accessible through only two controllers, the data bandwidth advantage of such arrays is generally lost. A single controller failure could overload the remaining controller, slowing system performance to unacceptable levels.
A clearly-felt need exists in the art for a redundant disk array architecture that allows continued operation without major performance degradation when one controller fails. There is also a need for such an architecture that defines parity groups independently of controller location, so that cost/performance objectives alone determine controller numbers independently of parity group size or number. These unresolved deficiencies are clearly felt in the art and solved by this invention in the manner described below.